Light olefins are an important basic chemical feedstock for the production of many plastics used in a variety of industries. Olefins are most commonly produced from petroleum feedstocks through the cracking of larger hydrocarbon molecules. The cracking process is either a catalytic or steam cracking process, and produces light olefins which consist primarily of ethylene and propylene.
An alternate source of light olefins is from the conversion of oxygenates to olefins. The primary oxygenate that is converted to an olefin is methanol. The preferred process is generally referred to as methanol-to-olefins (MTO) process. The primary olefins produced from this process are ethylene and propylene, and the process is performed over a catalytic molecular sieve. The MTO process enables an important alternative to petroleum sources of feeds for the production of light olefins. The sources of oxygenates include alcohols, such as methanol and ethanol; ethers, such as dimethyl ether and diethyl ether; and other oxygenates, such as methyl formate and dimethyl carbonate. These oxygenates can be produced from natural gas, fermentation of biomass, municipal wastes, and recycled organic materials. An important commercial consideration is that methanol can be readily produced from natural gas, or coal, and is easier and safer to handle and transport than either natural gas or coal.
There are numerous patents describing improved preparation of molecular sieves. U.S. Pat. No. 5,248,647 describes the hydrothermal treatment of silicoaluminophosphates molecular sieves at temperatures in excess of about 700° C. for periods sufficient to destroy a large proportion of their acid sites while retaining at least 80% of their crystallinity to form a catalyst for converting methanol to lower olefins. This catalyst shows increased catalyst life, increased selectivity for C2 to C3 olefins and decreased selectivity for paraffin production relative to the untreated molecular sieve. In U.S. Pat. No. 6,440,894, halogens are removed by steam-treating the catalyst at a temperature from 400° to 1000° C. The type of molecular sieve or catalyst, composition, size, and processing conditions affect the process of producing a high yield of light olefin and requires significant experimentation without providing guidance for specific molecular sieves.
The prior art teaches that catalyst exposure to water results in deactivation of the catalyst. For example, in U.S. Pat. No. 7,015,174, there is a lengthy explanation of the deactivation of the catalyst that is caused by exposure to water. Contrary to that patent's teachings, in the present invention it has been found that a treatment with water results in a catalyst that has an enhanced selectivity for the desired light olefins.
It is highly desirable to increase the yield of the desired light olefins—ethylene and propylene. Accordingly, it would be useful to produce a catalyst that produces a higher yield of these desired products.